Sigmund Freud may have been right after all— dreams are an open door to your unconscious mind.

You find yourself at a banquet table. You feel disaffected because the people surrounding you are speaking a language you do not understand. Suddenly, beneath the table, you feel someone's foot on top of your own. You glance up. Your eyes meet those of an attractive person and you sense there is one word that you now must say: "Phlegm." The person stands, and suddenly everyone else at the banquet is gone. As is the table. As are your clothes.

You fling yourselves at each other in passion. You rise up in the air, the sensuality of the experience heightened by clouds brushing past. Yet you begin to sob in shame because you have been observed by your four deceased grandparents, who disapprove. You suddenly realize that the severe-looking man in the black frock coat comforting your maternal grandmother is William Seward, and with great clarity and an inexplicable sense of nostalgia, you recite, "William Henry Seward, U.S. Secretary of State in the Andrew Johnson administration."You're dreaming.

To state a truism, just as the kidney could accurately be described as a kidney-shaped organ, dreams are dreamlike. Why should that be? In real life you wouldn't wind up floating amid the clouds with someone seconds after the touch of a foot. Instead, at such a moment you might remember you forgot to turn off the lights of your car. Dreams, by contrast, are characterized not only by rapid transitions but by a heightened sense of emotionality and irrationality. In dreams, you do things that with two seconds of sensible reflection you couldn't bring yourself to do in real life.

There has never been a shortage of theories about the utility of dreams being dreamlike. Maybe dreaming is the channel through which the gods choose to speak to mortals. Maybe it's the means to work out how you really feel about your mother without all that repression stuff getting in the way. Maybe it's a way to get your brain to work in an unconventional, orthogonal manner to solve that pesky math problem you went to sleep thinking about. Maybe it's how you keep underutilized neural pathways active by giving a workout to neurons that don't get exercised during the day. Or maybe the whole thing evolved so that the surrealists and dadaists could make a living.

How does your brain bring about this state of disinhibited imagery? Until recently, scientists understood little about the actual nuts and bolts of dreaming. But one thing we've known for some time is that there is a structure--an architecture, if you will--to sleep, with rhythmic cycles of deep, slow-wave sleep interspersed with the rapid-eye-movement (REM) sleep most associated with dreaming. And the levels of activity in the brain are not uniform throughout the stages of sleep. Techniques that indicate the overall levels of electrical activity in the brain have uncovered something intuitively obvious: During deep, slow-wave sleep, the average level of brain activity goes way down. This fits well with studies suggesting that the main purpose of slow-wave sleep is to allow for the replenishing of energy stores in the brain--the proverbial recharging of the batteries. But something very different happens during the onset of dreaming--a big increase in electrical activity. And this has a certain intuitive logic to it as well.

Advances in brain imaging technology now allow scientists to study activity and metabolism in small subregions of the brain rather than just the brain as a whole. In a pioneering series of studies, Allen Braun and his colleagues at the National Institutes of Health have taken a close look at the neuroanatomy of metabolism during sleep. In the process I think they may have uncovered the explanation for why dreams are so dreamlike.

The researchers utilized positron emission tomography, or PET scans, to measure the various rates of blood flow throughout the brain. One of the brain's remarkably adaptive features is that blood flow in a particular region will increase when that area increases its level of activity. In other words, there is a coupling between demand for energy and the supply of it. Thus, the extent of blood flow in a particular area of the brain can be used as an indirect index of the level of activity there. That is why PET scans, which easily show blood flow, are so helpful in this type of research.

Braun and crew got some volunteers who allowed themselves to be sleep-deprived for an ungodly 24 to 53 hours. Each bleary volunteer was eventually rolled into a PET machine and forced to stay awake while a baseline scan was made. Then, snug as a bug inside, each study subject was finally allowed to sleep while the scanning continued.

As the subjects slid into slow-wave sleep, the blood-flow changes observed made a lot of sense. Parts of the brain associated with arousal, known as the reticular activating system, shut down; ditto for brain regions involved in regulating muscle movement. Interestingly, regions involved in the consolidation and retrieval of memories did not have much of a decrease in blood flow, and hence metabolism. However, the pathways that bring information to and from those regions shut down dramatically, thus isolating them metabolically. The parts of the brain that first respond to sensory information had something of a metabolic shutdown, but the more dramatic changes were in downstream brain areas that integrate and associate those bytes of sensory information and give them meaning. The result: metabolically quiescent, sleeping brains.

While the scientists at the scanner's console bided their time, the sleeping subjects transitioned into REM sleep. And then the picture changed. Metabolic rates lept upward throughout subregions of the brain. Cortical and subcortical regions that regulate muscle movement and brain-stem regions that control breathing and heart rate all showed increases. In a part of the brain known as the limbic system, which is involved in emotion, there was an increase as well. The same was true for areas having to do with memory and sensory processing, especially those connected to vision and hearing.

Meanwhile, something subtle went on in the visual processing regions. The primary visual cortical region did not show much of an increase in metabolism, but there was a big jump in the downstream regions that integrate simple visual information. The primary visual cortical region is involved in the first steps of processing sight--the changing of patterns of pixels of light and dark into things like lines or curves. In contrast, the downstream areas are the integrators that turn those lines and curves into the perception of objects, faces, and scenes. Normally, an increase in activity in the downstream areas cannot occur without an increase in the primary areas. In other words, when you're wide awake, you can't get your eyes to see complex pictures without first going through an initial level of analysis. But REM sleep is a special case--you're not using the eyes. Instead, you're starting with the downstream integration of visual patterns. This, Braun and his colleagues have speculated convincingly, is what makes up the imagery of dreams.

So there are increases in metabolism during REM sleep in numerous parts of the brain. In some regions, metabolic rates even wind up being considerably higher than when someone is awake. But researchers have also found an exception that I think may be the answer to why dreams are dreamlike, in a region of the brain called the prefrontal cortex. Outside the prefrontal cortex, all of the brain regions most closely associated with the limbic system showed an increase in metabolism with the onset of REM sleep. Within the prefrontal cortex, however, only one of the four subregions increased in activity. The rest of those areas stayed on the floor of metabolic inactivity they had sunk to during the period of slow-wave sleep.

This is intriguing, given the functions of the prefrontal cortex. The human brain has many unique features when compared with an off-the-rack mammalian brain. Its sensory inputs and motor outputs are uniquely fine-tuned to make it possible to whip out an arpeggio on a piano. The limbic system allows for something virtually unprecedented among mammals: sexual receptivity among females throughout the reproductive cycle, not only at the time of ovulation. The vast cortex creates symphonies and calculus and philosophy, while the atypically numerous interconnections between the cortex and the limbic system allow for a particularly dreadful human attribute--the ability to think oneself into a depression.

Yet in many ways, the most remarkable feature of the human brain is the extent of the development and the power of that prefrontal cortex, the region that stays metabolically inactive during REM sleep. The prefrontal cortex is the brain region that plays a central role in self-discipline, in gratification postponement, in putting a rein on one's impulses. On the facetious level, this is the part of the brain that keeps you from belching loudly in the middle of a wedding ceremony or an important business meeting. On the more profound level, it keeps the angry thought from being allowed to become the hurtful word, the violent fantasy from becoming the unspeakable act.

Not surprisingly, other species don't have a whole lot of prefrontal function. Nor do young kids; the prefrontal cortex is basically the last part of the brain to fully mature, not coming completely online for decades. Violent sociopaths appear to have insufficient metabolic activity in the prefrontal region. And damage to the prefrontal cortex, such as that created by strokes, causes a disinhibited, frontal personality. The person may become apathetic or childishly silly, hypersexual or bellicose as hell, scatological or blasphemous.

Braun and his colleagues found that during REM sleep much of the prefrontal cortex was off-line, unable to carry out its waking task of censoring material, while there were high rates of activity in the complex sensory processing parts of the brain concerned with emotion and memories.

So bring on those dreams, now free to be filled with uninhibited actions and labile emotions.

You can breathe underwater, fly in the air, communicate telepathically; you can announce your love to strangers, invent languages, rule kingdoms; you can even star in a Busby Berkeley musical.

Mind you, even if it turns out that the lack of metabolic activity in the prefrontal cortex during REM sleep explains the disinhibition of dream content, it still doesn't tell us anything about why anyone's brain would spend time staging that particular musical. The specific content of dreams remains a mystery. Moreover, if true, this speculation would constitute one of the classic features of science--in explaining something, we've merely managed to redefine the unknown. Suppose the answer to the question "Why is dream content so disinhibited?" turns out to be "Because prefrontal cortical regions are atypically inactive during REM sleep." The new question obviously becomes "Then why are prefrontal cortical regions atypically inactive?"

Just as with anything else that can be studied and measured in living systems, there is considerable variability in the level of activity of the prefrontal cortex in different individuals. At one end of the spectrum, there seem to be decreased metabolic rates in prefrontal regions in sociopaths. At the other end of the spectrum, Richard Davidson, at the University of Wisconsin at Madison, and colleagues have observed elevated prefrontal metabolic rates in people with so-called repressive personalities. These are highly controlled folks, with superegos going full throttle, working overtime to keep their psychic sphincters good and tight. They dislike novelty, prefer structure and predictability, are poor at expressing emotions or at reading the nuances of emotions in other people. These are the folks who can tell you what they're having for dinner two weeks from Thursday.

This leads me to an idea that seems to flow naturally from the findings of Braun and his colleagues. The data regarding the sociopath/ repressive continuum come from studies of awake individuals. Most certainly, there will also be considerable variability among people as to how the prefrontal cortex functions during REM sleep. While prefrontal metabolism may remain on the floor with the transition into REM sleep on average, there will be exceptions. So I suspect it's likely that the more prefrontal metabolism remains suppressed during REM, the more vivid and disinhibited dream content will be, perhaps in a subject-specific manner. Most revealing would be some comparative studies of prefrontal metabolism during waking and sleep- ing periods. Do peo-ple who have the most active prefrontal cortices when awake have the least active when asleep? This would certainly fit the old hydraulic models of psychoanalysis, which postulate that if you repress something important during the day, it will most likely come oozing out during dreams.

At Stanford, where I direct a neuroscience lab, I've occasionally heard medical students come up with a witticism to express their disdain for classes in psychiatry. Question: "What classes are you taking this semester?" Answer: "Oh, pathology, microbio, pharmacology, and this required seminar in laser psychotherapy." The last is meant to be an eccentric oxymoron. Laser something-or-other equals high tech, as opposed to psychotherapy, the pejoratively low-tech art of talk therapy. Thus the student is saying: "They're forcing us to take some class with these shrinks who are trying to dress up their stuff as modern science." Wouldn't it be ironic if some reductive support for that seemingly antiquated Freudian concept of repression were to emerge from the bowels of a gazillion-dollar scanning machine?